1. Field of the Invention
The present invention relates generally to improvements in disc drive servo methods, and, more particularly, but not by way of limitation to improvements in methods for accessing tracks to which computer files are to be stored or from which a file is to be retrieved.
2. Brief Description of the Prior Art
In a disc drive for storing computer files, the files are stored along concentric, circular data tracks that are defined in magnetizable coatings on surfaces of rotating discs and the files are written, and subsequently read, by transducers that fly over the disc surfaces in close proximity to the surfaces. The tracks on different disc surfaces are organized into cylinders, each containing a track on one surface, and the transducers are mounted on an actuator that aligns the transducers so that the transducers all bear substantially the same spatial relationship to the cylinders. Thus, alignment of one transducer with a selected track results in substantial alignment of the other transducers with tracks in the same cylinder.
The actuator is pivotally supported on the disc drive case and comprises a coil that is immersed in a magnetic field, provided by permanent magnets on the drive case, so that currents supplied to the actuator coil will result in forces on the actuator that can be used to maintain alignment between the transducers and a selected cylinder or to move the transducers from a cylinder with which the transducers are presently aligned to a new cylinder which contains a target track from which a previously stored file is to be retrieved or to which a new file is to be written.
Conventionally, movements of the transducers between cylinders have been effected in two phases, a seek phase and an ensuing settle phase. The seek phase, with which the present invention is concerned, employs a velocity control approach in which the velocity of the transducers in each of a succession of time intervals is determined and compared to a velocity taken from a predetermined velocity profile that is expressed as a function of the distance to a target track and decreases to zero as the distance to the target track decreases to zero. Correction signals, each generated from the difference between the transducer velocity in each time interval and the profile velocity determined for that interval, are transmitted to an actuator driver that, ideally, passes a current through the actuator coil in proportion to the correction signal. Thus, during terminal portions of the seek in which the transducers are decelerated toward the cylinder containing the target track, the transducers are caused, ideally, to follow a trajectory in which the velocity of the transducers substantially follows the velocity profile toward a velocity of zero at the target cylinder; that is, the cylinder that contains the target track. (During initial portions of the seek in which the transducers are accelerated from initial tracks being followed at the time the seek commences, the velocity of the transducers will vary significantly from the velocity profile for reasons that will become clear below.) The settle phase, which begins shortly before the transducers reach the target cylinder, is then used to bring the transducers to rest at the target cylinder.
The advantage afforded by velocity control of the transducers during seeking is that, at least in principle, the time required to effect movement of the transducers can be minimized by appropriate design of the velocity profile and it will be useful to consider a particular profile design that has been used for this purpose. Initially, electrical power for the operation of the disc drive and, more particularly, the actuator driver is supplied by the computer with which the disc drive is used at, ideally, a design voltage such as 12 volts. Based upon this design voltage, the actuator driver can supply, again ideally, a maximum design current that depends upon the construction of the actuator driver and corresponds to a particular correction signal received by the actuator driver. For larger correction signals, the actuator driver will saturate to supply the maximum current of which it is capable.
Since the acceleration and deceleration of the transducers is substantially proportional to the current supplied by the actuator driver, the velocity profile can, in principle, be designed by requiring the deceleration of the transducers to be the maximum deceleration possible using the maximum current the actuator driver is designed to supply. This deceleration will be known from the construction of the actuator driver, the structure of the actuator coil and the locations and structures of the permanent magnets surrounding the actuator coil. Thus, the velocity profile can be generated by calculating the distance required to stop the transducers from each of a selection of velocities up to a maximum velocity the transducers are to have during the seek. The velocities and the distances are then stored; for example, as a look up table, as the portion of the velocity profile that corresponds to deceleration of the transducers. For distances greater than the distance required to stop the transducers from the maximum velocity they are to achieve, the profile velocities are selected to be such maximum velocity.
The manner in which such a velocity profile minimizes the time required for a seek can be seen by considering the manner in which a lengthy seek would generally, at least in principle, proceed using this velocity profile. At the time the seek begins, the velocity of the transducers will be zero while the profile velocity will be substantially the maximum velocity determined by the profile. Accordingly, the correction signal determined for the first time interval, and a number of succeeding time intervals, will generally exceed the maximum correction signal for which the actuator driver supplies a maximum current to the actuator coil. (That the correction signal will exceed the correction signal corresponding to maximum actuator coil current can be seen from the design of the velocity profile to provide maximum deceleration over a series of time intervals in which the transducer velocities substantially follow the velocity profile.) As a result, the actuator driver will saturate to provide the maximum current of which it is capable to the actuator coil to cause maximum acceleration of the transducers that will cause the transducer velocity to reach the velocity profile in a minimum time. Thereafter, the transducers will move at substantially the maximum speed they are to have during the seek until the distance from the target track at which deceleration from the maximum is to commence. The deceleration then occurs, because of the design of the profile described above, at the maximum rate determined by the maximum current that can be supplied by the actuator driver. Consequently, the transducers will move at the maximum speed selected for the seek or will be accelerated and decelerated at the maximum possible rate throughout the seek to minimize the time required for the seek.
In practice, seeking using a velocity profile determined using the design values of the voltage supplied to the actuator driver and the maximum design current the actuator driver can provide at this voltage is not feasible. The voltage supplied to the actuator driver varies with time to vary the maximum current that can be supplied to the actuator coil and the resistance of the coil similarly varies with time as currents passed through the coil in repeated transducer movements heat the coil. Consequently, the maximum design current cannot be reliably achieved with the result that the corresponding maximum deceleration of the transducers cannot be reliably achieved. Accordingly, should the profile velocity be designed using the maximum design current, the deceleration of the transducers would often occur at too small a rate for the transducers to closely follow the velocity profile. Thus, the transducers would commence settle with an excessive velocity that would, in many circumstances, cause excessive overshoot of the target cylinder that would prevent termination of the movement of the transducers at the target cylinder. That is, the transducers would be settled on the wrong cylinder. In such cases, a reseek of the target cylinder would be required to access the target track.
Because of these variations in the current supply capacity of the actuator driver, it is common practice to design the velocity profile using a nominal current that is a selected percentage of the maximum design current the actuator driver can supply. In the past, this nominal current has been selected on the basis of worst case conditions with the result that the time required to effect a seek has been unduly lengthened. In particular, to insure that the transducers can be settled on the target cylinder, the nominal current must generally be selected to be of the order of only 80 to 90 percent of the maximum design current. Since the time for the seek phase to be carried out depends upon the maximum acceleration and deceleration of the transducers, in turn depending upon the nominal current, the choice of a low nominal current must necessarily result in undesirable increases in transducer movement time.
Moreover, selection of higher nominal currents, subject only to the condition that settle on the target cylinder will occur, gives rise to a second problem. When the maximum current that can be supplied to the actuator coil is substantially less than this nominal current, even though such current is capable of effecting settle on the target cylinder, the excessive transducer velocity with which settle begins can result in an initial, relatively large, excursion of the transducers from the centers of the tracks of the target cylinder that will require an undesirably long time to correct. Thus, in general, selection of a relatively large nominal current to design a velocity profile that will minimize the time required for the seek will often result in a corresponding increase in settle time following the seek. Alternatively, attempts to minimize the settle time generally require velocity profile design using a lower nominal current that will increase the time required to move the transducers to the distance from the target track at which settle commences. Consequently, while the velocity control has proven to be an effective way of effecting the seek phase of transducer movements, such control has not, prior to the present invention, permitted such movements to be effected in an optimal time.